This invention relates in general to brake drums or rotors and in particular to a brake drum or rotor having improved fatigue resistance and friction pad burnishing characteristics.
Frictional forces are used to slow and stop wheeled vehicles that are in motion. Typically, non-rotating brake pads which are affixed to brake shoes or brake calipers are pressed against a surface which is attached to the wheel and rotating therewith. The rotating surface is formed on the inside of a brake drum or upon a brake rotor. As the brake pad slides over the surface, the wheel is slowed and the vehicle's kinetic energy is converted to heat. The heat is dissipated through the drum or rotor material. Finally, the brake pad clamps the surface, holding the attached wheel stationary.
Brake drums and rotors are often cast from iron, which has excellent heat conducting properties. The castings are machined to provide a true circular surface to be engaged by the brake pads. By truing the surface, vibration is minimized. Holes are also drilled in the casting which receive wheel mounting studs. In a typical application, vehicle wheels are bolted to the studs mounted in the brake drum. The brake drums are mounted upon an axle hub and rotate with the axle. Brake shoes having arcuate brake pads are mounted upon a backing plate. The backing plate is rigidly bolted to the vehicle housing axle and is thus held stationary. Disc brake rotors also have studs for attaching vehicle wheels and are mounted upon an axle hub the same as brake drums. However, the brake pads are held in position over machined rotor surfaces by calipers which are rigidly mounted upon the vehicle. Pressurized hydraulic or pneumatic actuators are used to urge the brake pads against the machined surface of the drum or rotor to brake the vehicle.
Repeated brake application cause cycles of heating and cooling of the brake drum which expand and contract the drum metal. The expansion and contraction of the metal causes increases in residual tensile stresses which were previously induced by the machining operations carried out upon the brake drum. As the tensile stresses increase, cracks may form in the drum surface. This formation of cracks in the brake drum surface is commonly referred to as heat checking.
While the brake pads are formed to the approximate curvature of the inside machined surface of the brake drum, the mating surfaces between new brake pads and new brake drums generally to do not match one another. However, the pads are burnished during initial usage on the vehicle to exactly match the brake drum surface. During this initial burnishing, high spots on the brake pad can cause localized areas of high temperature on the inside machined surface of the brake drum. These high surface temperatures can change the crystalline structure of the drum metal as the cast iron is changed to martensite. This is known as hot spotting and can cause surface cracking.
Disc brake rotors are subject to the same heating and cooling effects of repeated braking cycles.
In the past, it has been known that residual tensile stresses induced in a metal part during machining operations can be counteracted by subjecting the part to a prestressing process to induce compressive stresses in the surface of the part. If the induced compressive stress exceeds the tensile stress, a residual compressive stress is then established in the surface of the part. The residual compressive stress acts to offset service-imposed tensile stress, such as that caused by heat checking, and significantly increases the fatigue life of the part. Processes which have heretofore been used to induce compressive stresses include surface hardening, honing, polishing, burnishing, rolling and shot peening. Shot peening, in particular, has been found effective in treating many vehicle parts such as connecting rods, crankshafts, gears, shafts, axles, coil and leaf springs.